The present invention relates to a method and apparatus for setting a gap between first and second objects to a predetermined distance and, more particularly, a method and apparatus for setting a gap between a mask and a wafer to a predetermined distance when the image of a circuit pattern is transferred to a wafer.
In the process of manufacturing a semiconductor device such as a VLSI, a circuit pattern is normally exposed on a wafer, by means of an exposure apparatus. Using this apparatus, a circuit pattern preformed on a mask is irradiated with X-rays, which causes an image of the circuit pattern to be transferred to a wafer. Before the circuit pattern is transferred, a gap between the mask and the wafer must be accurately set to a predetermined distance.
A method for setting a gap between a mask and a wafer is disclosed in Japanese Patent Disclosure (Kokai) No. 61-116837. In this method, a diffraction grating is used. More specifically, as is shown in FIG. 1, one-dimensional diffraction grating 2 is formed on mask 1, and reflection surface 4 is formed of wafer 3. When laser beam is radiated onto the upper surface of mask 1, light beams diffracted by and passing through diffraction grating 2 of mask 1 are reflected by reflection surface 4 of wafer 3, and are diffracted again by diffraction grating 2 of mask 1. Among these diffracted light beams, diffracted light beam I.sub.n (+1) of the +1st order and diffracted light beam I.sub.n (-1) of -1st order are measured. The result of this measurement is indicated by the broken curve in FIG. 2. More specifically, the relationship between the intensity of diffracted light and the distance of a gap corresponds to a periodic function having a period of P.sup.2 /.lambda. (when P is the pitch of diffraction grating 2, and .lambda. is a wavelength of laser beams). The distance of a gap between mask 1 and wafer 3 is adjusted so that intensity I.sub.n of diffracted light beams corresponds to the peak value of the periodic function. Thus, a gap between mask 1 and wafer 3 can be set to a predetermined value.
However, the diffraction grating of mask 1 also serves as a reflector type diffraction grating. For this reason, the diffracted light beams of the .+-.1st orders, which are diffracted along the path of mask 1.fwdarw.wafer 3.fwdarw.mask 1, interfere with the reflected diffracted light beam of the 1st order, reflected by the upper surface of the mask. If a distance of a gap between the mask and the wafer is z, the diffracted light beams of the +1st orders have an optical path difference of 2z, with respect to the reflected diffracted light beam of the 1st order. If 2z=n.lambda. (n is an integer), the diffracted light beams of the .+-.1st orders interfere with the reflected diffracted light beams of the 1st order. For this reason, as is shown in FIG. 2, the relationship between the intensity of diffracted light beams and the distance of the gap becomes a nonuniform periodic function having a period .lambda./2. Consequently, it is then difficult to adjust the size of a gap between the mask and the wafer.